Method and system for determining symbol boundary timing in a multicarrier data transmission system

ABSTRACT

Improved techniques for acquiring symbol boundary timing at a receiver of a multicarrier data transmission system during a training sequence are disclosed. One aspect is symbol boundary determination at a receiver wherein minimal interference is used as a criterion in selecting from a plurality of potential symbol boundary timings. The symbol boundary determination at the receiver can be performed in a time domain or a frequency domain manner. Another aspect pertains to an improved training sequence wherein pairs of identical symbols are transmitted by a transmitter. These symbols can be supplied to the transmitter in a time domain or a frequency domain manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to: (i) U.S. patent application Ser. No.______, filed concurrently, entitled “TRAINING SEQUENCE FOR SYMBOLBOUNDARY DETECTION IN A MULTICARRIER DATA TRANSMISSION SYSTEM,” andincorporated herein by reference; (ii) U.S. patent application Ser. No.______, filed concurrently, entitled “TRAINING SEQUENCE FOR CHANNELESTIMATION IN A DATA TRANSMISSION SYSTEM,” and incorporated herein byreference; and (iii) U.S. patent application Ser. No. ______, filedconcurrently, entitled “METHOD AND SYSTEM FOR CHANNEL ESTIMATION IN ADATA TRANSMISSION SYSTEM,” and incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to data transmission systems and, moreparticularly, to multicarrier data transmission systems.

2. Description of the Related Art

A conventional voice-band modem can connect computer users end-to-endthrough the Public Switched Telephone Network (PSTN). However, thetransmission throughput of a voice-band modem is limited to below 40Kbps due to the 3.5 KHz bandwidth enforced by bandpass filters and codesat the PSTN interface points. On the other hand, the twisted-pairtelephone subscriber loop of a computer user has a much wider usablebandwidth. Depending on the length of the subscriber loop, the bandwidthat a loss of 50 dB can be as wide as 1 MHz. Transmission systems basedon local subscriber loops are generally called Digital Subscriber Lines(DSL).

As consumer demand has increased for interactive electronic access toentertainment (e.g. video-on-demand) and information (Internet) indigital format, this demand has effectively exceeded the capabilities ofconventional voice-band modems. Various delivery approaches have beenproposed, such as optical fiber links to every home, direct satellitetransmission, and wideband coaxial cable. However, these approaches areoften too costly. Hence, cheaper alternatives have emerged, such as thecable modem which uses existing coaxial cable connections to homes andvarious high bit rate DSL modems which use the existing twisted-pair ofcopper wires connecting a home to the telephone company's central office(CO).

One DSL technique for high-speed data communications is AsymmetricalDigital Subscriber Line (ADSL) signaling for the telephone loop that hasbeen defined by standards as a communication system specification thatprovides a low-rate data stream from the residence to the CO (upstream),and a high-rate data stream from the CO to the residence (downstream).The ADSL standard provides for operation without affecting conventionalvoice telephone communications, e.g., Plain Old Telephone Service(POTS). The ADSL upstream channel only provides simple control functionsor low-rate data transfers. The high-rate downstream channel provides amuch higher throughput. This asymmetrical information flow is desirablefor applications such as video-on-demand (VOD).

An ADSL modem operates in a frequency range that is higher than thevoice-band; this permits higher data rates. However, the twisted-pairsubscriber line has distortion and losses which increase with frequencyand line length. Thus, the ADSL standard data rate is determined by amaximum achievable rate for a length of subscriber lines.

The ADSL standard uses Discrete Multi-Tone (DMT) modulation with the DMTspectrum divided into two-hundred fifty-six 4.3125 KHz carrier bands anda quadrature amplitude modulation (QAM) type of constellation is used toload a variable number of bits onto each carrier band independently ofthe other carrier bands.

The number of bits per carrier is determined during a training periodwhen a test signal is transmitted through the subscriber line to thereceiving modem. Based on the measured signal-to-noise ratio of thereceived signal, the receiving modem determines the optimal bitallocation, placing more bits on the more robust carrier bands, andreturns that information back to the transmitting modem.

The modulation of the coded bits is performed very efficiently by usinga 512-point inverse fast Fourier transform (IFFT) to convert thefrequency domain coded bits into a time domain signal which is put onthe twisted-pair by a Digital-to-Analog (D/A) converter using a samplerate of 2.208 MHz (4.3125×512). The receiving ADSL modem samples thesignal and recovers the coded bits with a fast Fourier transform (FFT).

A typical DMT system utilizes a transmitter inverse FFT and a receiverforward FFT. Ideally, the channel frequency distortion can be correctedby a frequency domain equalizer following the receiver FFT. However, thedelay spread of the channel in the beginning of the receiver FFT blockcontains inter-symbol interference from the previous block. As thisinterference is independent of the current block of data, it cannot becanceled just by the frequency domain equalizer. The typical solutionadds a block of prefix data in front of the FFT data block on thetransmitter side before the block of FFT data is sent to the D/Aconverter. The prefix data is the repeat copy of the last section of FFTdata block.

On the receiver side, the cyclic prefix is removed from the receivedsignal. If the length of the channel impulse response is shorter thanthe prefix length, inter-symbol interference from the previous FFT datablock is completely eliminated. Frequency domain equalizer techniquesare then applied to remove intra-symbol interference among DMTsubchannels. However, since the channel impulse response varies on acase by case basis, there is no guarantee that the length of the impulseresponse is shorter than the prefix length. An adaptive time domainequalizer is typically required to shorten the length of the channelresponse within the prefix length.

Time domain equalizer training procedures have been studied previously,see “Equalizer Training Algorithms for Multicarrier Modulation Systems,”J. S. Chow, J. M. Cioffi, and J. A. C. Bingham, 1993 InternationalConference on Communications, pages 761-765, Geneva, May 1993. Acorresponding training sequence has also been specified in “ADSLstandard and Recommended Training Sequence for Time domain Equalizers(TQE) with DMT,” J. S. Chow, J. M. Cioffi, and J. A. C. Bingham, ANSIT1E1.4 Committee Contribution number 93-086.

Besides ADSL, another DSL technique for high-speed data communicationsover twisted-pair phone lines is known as Very High Speed DigitalSubscriber Lines (VDSL). VDSL is intended to facilitate transmissionrates greater than that offered by the ADSL. The multi-carriertransmission schemes used with VDSL can be Discrete Multi-Tone (DMT)modulation, or some other modulation scheme such as Discrete WaveletMulti-Tone (DWMT) modulation, Quadrature Amplitude Modulation (QAM),Carrierless Amplitude and Phase modulation (CAP), Quadrature Phase ShiftKeying (QPSK), or vestigial sideband modulation.

A common feature of the above-mentioned transmission systems is thattwisted-pair phone lines are used as at least a part of the transmissionmedium that connects a central office (e.g., telephone company) to users(e.g., a residence or business). Even though fiber optics may beavailable from a central office to the curb near a users residence,twisted-pair phone lines are used to bring the signals from the curbinto the user's home or business.

One conventional frame synchronization technique for a system, usingfrequency division duplexing (FDD) or echo cancelling to provideduplexed operation, required the transmission of a predeterminedsequence of data which was received by a receiver and then correlatedwith a predetermined stored sequence of data to determine the adjustmentrequired in order to yield synchronization. This frame synchronizationtechnique requires a special start-up training sequence to obtain theframe synchronization.

When a data transmission system is operating in a time-division duplexed(TDD) manner, the transmitters and receivers of the central office andremote units must be synchronized in time so that transmission andreception do not overlap in time. In a data transmission system,downstream transmissions are from a central side transmitter to one ormore remote side receivers, and upstream transmissions are from one ormore remote side transmitters to a central side receiver. The centralside transmitter and receiver can be combined as a central sidetransceiver, and the remote side transmitter and receiver can becombined as a remote side transceiver.

Generally speaking, in a time-division duplexed system, upstream signalsare alternated with downstream signals. On channels subject to crosstalk(NEXT interference) between multiple connections, if time-divisionduplexing is used, synchronization must be established and maintainedamong all units so affected. Typically, the upstream transmissions andthe downstream transmissions are separated by a guard interval or aquiet period. The guard interval is provided to enable the transmissionsystem to reverse the direction in which data is being transmitted sothat a transmission can be received before the transmission in theopposite direction occurs. Some transmission schemes divide upstream anddownstream transmissions into smaller units referred to as frames. Theseframes may also be grouped into superframes that include a series ofdownstream frames and a series of upstream frames, as well as guardintervals between the two.

In multi-carrier data transmission systems, high speed data transfer canbe performed using a plurality of sub-carriers. A Discrete Multi-Tone(DMT) symbol is transmitted using the plurality of sub-carriers. Acyclic prefix is inserted to maintain the circularity of DMT symbols. Acyclic prefix is formed by adding the last several samples to thebeginning of a DMT symbol. The length of the cyclic prefix can berepresented as Lcp. Likewise, a cyclic suffix can also be used for thesame purpose and it is formed by adding the first few samples to the endof a DMT symbol. The length of the cyclic suffix can be represented asLcs.

In addition, the cyclic suffix or cyclic prefix can be used for aligningthe DMT transmit and receive windows in digital duplexing. See, e.g.,John M. Cioffi et al. “G.vdsl: Digital Duplexing: VDSL PerformanceImprovement by Aversion of Frequency Guard Bands,” ITU TemporaryDocument NT-041, Nashville, Tenn., November 1999, which is incorporatedherein by reference. Although the cyclic prefix is used as the mechanismto ensure circularity, the same principle applies to cyclic suffixes.Similarly, the cyclic suffix is used for aligning symbols in digitalduplexing, but the same purpose can be fulfilled by either the cyclicprefix or the cyclic suffix, or a combination of both.

FIG. 1 is a diagram illustrating a representative, conventional DMTsymbol 10 having a cyclic prefix 12 of length Lcp and a cyclic suffix 14of length Lcs. A transmitter window is often applied to DMT symbols andthe last β samples of the cyclic suffix of the preceding DMT symbol andthe first β samples of the cyclic prefix of the latter DMT symbol areoverlapped and added. The resulting length of the DMT symbol is thusdefined asdmtsymbol_length=2*Nsc+Lcp+Lcs−β  (Eq. 1)where Nsc is the number of sub-carriers.

In determining the symbol boundary timing for multicarrier systems suchas OFDM (orthogonal frequency division multiplexing—a generalization ofDMT systems), a conventional method is described in L. Hanzo et al.,“Single- and Multi-carrier Quadrature Amplitude Modulation,” Wiley, ISBN0471492396, which is hereby incorporated herein by reference. Byexploiting the cyclic prefix structure, a cross-correlation function iscomputed. The symbol delay index is chosen to maximize such a function.That is, the cross-correlation function, GO), is formed with$\begin{matrix}{{G(j)} = {\sum\limits_{n = 0}^{{Lcp} - 1}\quad{{r\left( {j + n} \right)} \cdot {r\left( {j + n + {2 \cdot N_{sc}}} \right)}}}} & \left( {{Eq}.\quad 2} \right)\end{matrix}$where r refers to received samples. Further, the delay index, m_(d), isselected to be x such that $\begin{matrix}{{G\left( m_{d} \right)} = {\max\limits_{x}{\left\{ {G(x)} \right\}\quad{for}\quad{all}\quad x}}} & \left( {{Eq}.\quad 3} \right)\end{matrix}$

Accordingly, the conventional method seeks a symbol delay index tomaximize the received signal energy. However, these high-speed datacommunication systems are subject to interference, e.g., inter-symbolinterference and/or inter-carrier interference, which hinders operation.Inter-symbol interference is due to imperfections in the subscriber loop(i.e., non-ideal channel response), whereas inter-carrier interferenceis interference from one subcarrier to another. With longer loops orgreater channel spread, the impact of such interference tends to worsen.Thus, there is always a need for improved approaches to determinate ofappropriate symbol boundaries so that greater performance can beachieved.

SUMMARY OF THE INVENTION

Broadly speaking, the invention pertains to improved techniques foracquiring symbol boundary timing at a receiver of a multicarrier datatransmission system during a training sequence. One aspect is symbolboundary determination at a receiver wherein minimal interference isused as a criterion in selecting from a plurality of potential symbolboundary timings. The symbol boundary determination at the receiver canbe performed in a time domain or a frequency domain manner. Anotheraspect pertains to an improved training sequence wherein pairs ofidentical symbols are transmitted by a transmitter. These symbols can besupplied to the transmitter in a time domain or a frequency domainmanner. Different embodiments of the invention can incorporate one ormore of these aspects.

The invention can be implemented in numerous ways, including as amethod, system, device, apparatus, graphical user interface, or computerreadable medium. Several embodiments of the invention are discussedbelow.

As a method for detecting received symbol boundary timing in amulticarrier system, one embodiment of the invention includes at leastthe acts of: estimating noise due to inter-carrier and inter-symbolinterference for different choices of symbol boundary timing; andselecting the choice of symbol boundary timing with the least noise dueto inter-carrier and inter-symbol interference.

As a method for detecting received symbol boundary timing in amulticarrier system, another embodiment of the invention includes atleast the acts of: receiving a series of received training signals overa wire-based channel; storing at least three of the series of receivedtraining signals to a buffer; determining difference values for a pairof consecutive received training signals stored in the buffer; selectingone of the difference values; and determining the received symbolboundary timing based on the selected one of the difference values.

As a method for detecting received symbol boundary timing in amulticarrier system, yet another embodiment of the invention includes atleast the acts of: receiving a series of received training signals overa wire-based channel; determining a plurality of correlation values fora plurality of pairs of received training signals, where each respectivepair of training symbols were identical when transmitted; selecting oneof the correlation values; and determining the received symbol boundarytiming based on the selected one of the correlation values.

As a multicarrier data transmission system one embodiment of theinvention includes at least one transmitter and at least one receiver.The transmitter includes at least: a line encoder that encodes data intosymbols to be transmitted; training symbols; a switch that selectssymbols for transmission either the training symbols or the symbolspertaining to the encoded data from the line encoder; a multicarriermodulation unit for modulating the symbols into modulated symbols; and adigital-to-analog converter for converting the modulated symbols intoanalog signals. The receiver includes at least: an analog-to-digitalconverter for converting the analog signals into digital samples; asymbol timing unit for examining the analog signals to producecorrelation values for a plurality of different potential symbol timingboundaries; a multicarrier demodulation unit for demodulating thedigital samples into symbols; and a line decoder that decodes thesymbols into data. The transmitter transmits pairs of identical trainingsymbols, and the symbol timing unit produces the correlation values withrespect to the pairs of identical training symbols as received at thereceiver.

As a computer readable medium including at least computer program codefor detecting received symbol boundary timing in a multicarrier system,one embodiment of the invention includes at least: computer program codefor estimating noise due to inter-carrier and inter-symbol interferencefor different choices of symbol boundary timing; and computer programcode for selecting the choice of symbol boundary timing with the leastnoise due to inter-carrier and inter-symbol interference.

Other aspects and advantages of the invention will become apparent fromthe following detailed description taken in conjunction with theaccompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be readily understood by the following detaileddescription in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 is a diagram illustrating a representative, conventional DMTsymbol having a cyclic prefix and a cyclic suffix.

FIG. 2A is a block diagram of a transmitter according to one embodimentof the invention.

FIG. 2B is a block diagram of a receiver according to one embodiment ofthe invention.

FIG. 3A is a flow diagram of a training sequence process according toone embodiment of the invention.

FIG. 3B is a flow diagram of a training sequence process according toanother embodiment of the invention.

FIG. 3C is a flow diagram of a training sequence process according tostill another embodiment of the invention.

FIG. 4 is a diagram of an initial portion of a representative trainingsequence according to one embodiment of the invention.

FIGS. 5A-5C illustrate representative pairs of identical symbols thatcan be utilized according to different embodiments of the invention.

FIG. 6 is a flow diagram of a symbol boundary determination processaccording to one embodiment of the invention.

FIGS. 7A-7B are flow diagrams of a symbol boundary determination processaccording to another embodiment of the invention.

FIG. 8 is a block diagram of a symbol boundary timing unit according toone embodiment of the invention.

FIG. 9 is block diagram of a symbol boundary determination systemaccording to one embodiment of the invention.

FIG. 10 is a block diagram of a symbol boundary determination systemaccording to another embodiment of the invention.

FIG. 11 shows an exemplary graph of correlation values plotted againstthe symbol delay indices.

FIG. 12 illustrates a block diagram of an initial section of atransmitter according to one embodiment of the invention.

FIG. 13 illustrates a pair of representative training symbols in thefrequency domain according to one embodiment of the invention.

FIG. 14 is a block diagram of a symbol boundary determination systemaccording to another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention pertains to improved techniques for acquiring symbolboundary timing at a receiver of a multicarrier data transmission systemduring a training sequence. One aspect is symbol boundary determinationat a receiver wherein minimal interference is used as a criterion inselecting from a plurality of potential symbol boundary timings. Thesymbol boundary determination at the receiver can be performed in a timedomain or a frequency domain manner. Another aspect pertains to animproved training sequence wherein pairs of identical symbols aretransmitted by a transmitter. These symbols can be supplied to thetransmitter in a time domain or a frequency domain manner. Differentembodiments of the invention can incorporate one or more of theseaspects.

Embodiments of the invention are discussed below with reference to FIGS.1-14. However, those skilled in the art will readily appreciate that thedetailed description given herein with respect to these figures is forexplanatory purposes as the invention extends beyond these limitedembodiments.

FIG. 2A is a block diagram of a transmitter 100 according to oneembodiment of the invention. The transmitter 100 is, for example,associated with a multicarrier data transmission system.

The transmitter 100 includes a switch 102. The switch 102 can provideone data path in a data transmission mode and another data path in atraining mode. When the switch 102 is set to the training mode, trainingsymbols 104 are supplied to the switch 102. In addition, a trainingprotocol control unit 106 can interact with the switch 102 so as tocontrol the manner by which the training symbols 104 are transmitted. Inone implementation, the training protocol control unit 106 controls theswitch 102 such that the training symbols 104 are transmitted so thatpairs of identical training symbols are transmitted. In one example,each pair of identical training symbols being transmitted can beconsecutive.

On the other hand, when the switch 102 is in the data transmission mode,data 108 to be transmitted is supplied to a line encoder 110. The lineencoder encodes the data 108 into symbols which are then supplied to theswitch 102. For example, the symbols can be Quadrature AmplitudeModulated (QAM) symbols.

The switch 102 outputs the appropriate symbols, i.e., data transmissionsymbols or training symbols. The symbols output from the switch 102 aresupplied to a Discrete Multi-Tone (DMT) modulation unit 112 thatconverts the symbols into DMT symbols 114. For example, the DMTmodulation unit 112 can be an Inverse Fast Fourier Transform (IFFT)circuit. The DMT symbols 114 are supplied to a cyclic prefix/suffixinsertion unit 116. The cyclic prefix/suffix insertion unit inserts acyclic prefix and/or a cyclic suffix to the DMT symbols 114. The cyclicprefix and/or cyclic suffix is used to maintain the circularity of DMTsymbol. The outputs of the cyclic prefix insertion unit 116 are enhancedsymbols 118. The transmitter 100 optionally includes a windowingfunction 120. The windowing function 120 can be imposed on the enhancedsymbols 118 to produce modified symbols 122. The windowing function 120can further reduce interference by reducing the magnitude of thesidelobes at the receiver, and/or by reducing out-of-band energy at atransmitter. The modified symbols are then supplied to an analog frontend 124 which converts the modified symbols 122 from a digital form intoan analog form and thus outputs analog signals 126 to a wire where theyare transmitted to one or more receivers. The analog front end, forexample, includes one or more line drivers, filters anddigital-to-analog converters.

FIG. 2B is a block diagram of a receiver 200 according to one embodimentof the invention. The receiver 200 is, for example, associated with amulticarrier data transmission system. The receiver 200 is suitable forreceiving data transmitted by the transmitter 100 illustrated in FIG.2A.

The receiver 200 receives incoming analog signals 202 from a line. Forexample, the incoming analog signals 202 may have been transmitted tothe receiver 200 over the line by the transmitter 100 illustrated inFIG. 2A. The incoming analog signals 202 being received are supplied toan analog front end 204. The analog front end produces digital samples206. The analog front end 204 typically includes one or more of:amplifiers, filters and analog-to-digital converters. The digitalsamples 206, for example, pertain to DMT samples.

The digital samples 206 are supplied to a symbol timing unit 208. Thesymbol timing unit 208 examines the digital samples 206 and determinesan appropriate symbol boundary for recovery of the transmitted data. Inone embodiment, the symbol timing unit 208 determines the appropriatesymbol boundary timing based on interference values at differentpossible symbol boundaries. For example, those of the possible symbolboundaries having the lowest interference values would be the desirablechoices for the appropriate symbol boundary timing. Once the appropriatesymbol boundary timing has been determined, the symbol timing unit 208can provide control signals 211 to various other components within thereceiver 200 so as to synchronize the data recovery with the symboltiming boundaries. The symbol timing unit 208 will operate during atraining sequence or training mode in which the correspondingtransmitter (e.g., the transmitter 100 shown in FIG. 2A) transmitstraining symbols (e.g., training symbols 104) to the receiver 200.

The digital samples 206 are also supplied to a Time domain EQualizer(TEQ) 210. The time domain equalizer 210 outputs enhanced symbols 212that are supplied to a cyclic prefix removal unit 214. The TEQ 210shortens the effective channel spread so that the effective channel(actual channel+TEQ) has a spread that is shorter than the cyclic prefix(and/or cyclic suffix). The cyclic prefix/suffix removal unit 214removes any cyclic prefix and/or cyclic suffix that may be attached tothe enhanced symbol 212 and could optionally provide receiver windowing.The cyclic prefix removal unit 214 outputs DMT symbols 216. A DMTdemodulation unit 218 receives the DMT symbols 216 and outputsQuadrature Amplitude Modulated (QAM) symbols 220. The QAM symbols 220result from a demodulation of the DMT symbols 216. In oneimplementation, the DMT demodulation unit 218 can include a Fast FourierTransform (FFT).

The QAM symbols 220 can further be modified to reduce interference dueto nearby or far away sources. For example, the receiver 200 canoptionally include an adder 222 and an echo and NEXT canceller 224. Theadder 222 receives a next estimate 225 from the echo and NEXT canceller224. The adder 222 subtracts the next estimate 225 from the QAM symbols220 to produce modified QAM symbols 226. Furthermore, the receiver 200can optionally include a FEXT canceller 228. The modified QAM symbols226 can be supplied to the FEXT canceller 228 so that far awayinterference can be cancelled from the modified QAM symbols 226. Theoutput of the FEXT canceller 228 are modified QAM symbols 230. Finally,a line decoder 232 can decode the modified QAM symbols 230 into decodeddata 234 that is output from the receiver 200. The output from thereceiver 200 should be the digital data that was originally transmittedto the receiver 200.

The invention is particularly concerned with transmitting trainingsymbols from a transmitter, and then determining a symbol boundarytiming at a receiver in accordance with information gathered using thereceived training symbols at the receiver. The receiver is not requiredto have any knowledge of the training symbols or a sequence thereof.

FIG. 3A is a flow diagram of a training sequence process 300 accordingto one embodiment of the invention. The training sequence process 300initially obtains 302 training symbols. The training symbols are symbols(e.g., QAM symbols) that are to be transmitted using multicarriermodulation techniques to one or more receivers in order to permit theone or more receivers to acquire the symbol boundary timing needed toproperly decode the data that is to be transmitted to the one or morereceivers.

The training symbols can be obtained 302 in a number of different ways.For example, in one embodiment, the training symbols can be obtained 302by generating the training symbols. Typically, the training symbols areproduced in a pseudo-random manner. In one embodiment, the trainingsymbols can be generated by a pseudo-random generator. In anotherembodiment, the training symbols can be pre-stored in memory (e.g., alook-up table) and then obtained 302 by reading the appropriate trainingsymbols from the memory.

After the training symbols have been obtained 302, a first trainingsymbol is selected 304. The selected training symbol is then transmitted306 twice in succession. Next, a decision 308 determines whether thereare more training symbols to be transmitted. The number of trainingsymbols can be configurable by manufacturers or users, depending on theneeds of specific applications. In any case, when the decision 308determines that more training symbols are to be transmitted, thetraining sequence process 300 returns to repeat the block 304 so that anext training symbol can be selected and then subsequently transmitted306 twice in succession. Hence, the various training symbols typicallyrepresent a series of training symbols that are to be transmitted. Inthis embodiment, the training symbols are transmitted so that eachsymbol in the series is transmitted twice in succession. In any case,when the decision 308 determines that there are no more training symbolsto be transmitted, the training sequence process 300 is complete andends.

In practice, the training sequence described by FIG. 3A is an efficientembodiment of a training sequence process according to the invention.However, the training sequence process can be generalized so thattraining symbols are transmitted 306 one or more times. For example,besides transmitting training symbols twice in succession, the trainingsymbols could be transmitted once or three times or more in succession.

FIG. 3B is a flow diagram of a training sequence process 340 accordingto another embodiment of the invention. The training sequence process340 initially obtains 342 training symbols. The training symbols aresymbols that are to be transmitted to one or more receivers in order topermit the one or more receivers to acquire the symbol boundary timingneeded to properly decode the data that is to be transmitted to the oneor more receivers. Typically, the training symbols are transmitted usingmulticarrier modulation techniques to the one or more receivers. Aspreviously noted, the training symbols can be obtained 342 in a numberof different ways. After the training symbols have been obtained 342,the training symbols are transmitted 344 to the one or more receivers.The manner in which the training symbols are transmitted is such that atleast two identical symbols are preceded or succeeded by differentsymbols.

FIG. 3C is a flow diagram of a training sequence process 380 accordingto still another embodiment of the invention. The training sequenceprocess 380 represents, like the training sequence process 300,represent embodiments of the training sequence process 340. The trainingsequence process 380 also represents a generalization of the trainingsequence process 300 in that training symbols can be repeated zero ormore time during transmission.

The training sequence process 380 initially obtains 382 trainingsymbols. The training symbols are symbols that are to be transmitted toone or more receivers in order to permit the one or more receivers toacquire the symbol boundary timing needed to properly decode the datathat is to be transmitted to the one or more receivers. Typically, thetraining symbols are transmitted using multicarrier modulationtechniques to the one or more receivers. As previously noted, thetraining symbols can be obtained 382 in a number of different ways. Thetraining symbols may or may not be distinct from one another. After thetraining symbols have been obtained 382, a first training symbol isselected 384. The selected training symbol is then transmitted 386. Adecision 388 then determines whether the selected training symbol is tobe repeated. When the selected training symbol is to be repeated, theselected training symbol is again transmitted 386. Next, a decision 390determines whether there are more training symbols to be transmitted.The number of training symbols can be configurable by manufacturers orusers, depending on the needs of specific applications. In any case,when the decision 392 determines that more training symbols are to betransmitted, the training sequence process 380 returns to repeat theblock 384 so that a next training symbol can be selected and thensubsequently transmitted 386, 390 one or more times. Hence, the varioustraining symbols typically represent a series of training symbols thatare to be transmitted. In this embodiment, the training symbols aretransmitted so that each symbol in the series is transmitted one or moretimes in succession. In any case, when the decision 392 determines thatthere are no more training symbols to be transmitted, a decision 392determines whether at least two identical symbols have been preceded orsucceeded by different symbols in the course of the transmission 386 ofthe series of training symbols. When the decision 392 indicates that notwo identical symbols have been preceded or succeeded by differentsymbols, the training sequence process 380 returns to repeat the block384 so that the processing can continue so that this condition can besatisfied. On the other hand, when the decision 392 indicates that atleast two identical symbols have been preceded or succeeded by differentsymbols, the training sequence process 380 is complete and ends.

The particular transmission sequence of training symbols by variousembodiment of the training sequence process 300, 340 and 380 can vary. Afirst example is the transmission sequence “A, A, B, B, C, C, D, D”which allows symbols A, B, C and D to all be used in acquiring symbolboundary timing. The training sequence process 300 of FIG. 3A isdesigned to use such an efficient approach. The training sequenceprocesses 340 and 380 of FIGS. 3B and 3C are also able to utilize thetransmission sequence of the first example as well as many others, suchas exemplified by the following examples. A second example is thetransmission sequence “A, B, B, C, D, E, E, F, F, F, C, G” which allowssymbols B, C, E and F to be used in acquiring symbol boundary timing. Athird example is the transmission sequence “A, B, C, D, E, B, F” whichallows symbol B to be used in acquiring symbol boundary timing. A fourthexample is the transmission sequence “A, B, C, B, C, D” which allowssymbols B and C to be used in acquiring symbol boundary timing. A fifthexample is the transmission sequence “A, B, D, E, B, D, F” which allowssymbols B and D to be used in acquiring symbol boundary timing. To theextent that a transmission sequence uses symbols that cannot be used inacquiring symbol boundary timing, such symbols increase the latency toacquire symbol boundary timing, thus are less efficient. However, insome cases, the symbols being transmitted but not used to acquire symbolboundary timing can be used for other purposes. It should be understoodthat these exemplary transmission sequences are abbreviated for ease ofillustration, as typically training sequences will be substantiallylonger (e.g., 500 usable symbol pairs)

FIG. 4 is a diagram of an initial portion of a representative trainingsequence 400 according to one embodiment of the invention. As shown inFIG. 4, the training sequence 400 has an initial portion thatillustrates the first six symbols to be transmitted. Note that the firstand second symbols that are transmitted are both symbol A, that thethird and fourth symbols transmitted are both symbol B, and that thefifth and sixth symbols transmitted are both symbol C. In other words,each symbol of the training sequence 400 is sent twice, one after theother. The training sequence process 300 can be used to produce thetraining sequence 400. The advantages of this particular type oftraining sequence and transmission of such training symbols is explainedin more detail below.

FIGS. 5A-5C illustrate representative pairs of identical symbols thatcan be utilized according to different embodiments of the invention.FIG. 5A illustrates a pair of identical symbols 500 according to oneembodiment of the invention. The pair of identical symbols includesymbol-0 502 and symbol-1 504. The symbol-0 502 is initially transmittedand then immediately followed by the transmission of symbol-1 504. Boththe symbol-0 502 and the symbol-1 504 pertain to the transmission ofsymbol A. Hence, the symbol-0 502 and the symbol-1 504 can represent apair of identical symbols. The symbol A includes a plurality of samplesA0-A17 which represent data values transmitted on each of a plurality ofsubcarriers utilized by the multicarrier data transmission system. Thesample numbers used in FIGS. 5A-5C (i.e., A0-A17, A0-A13, A0-A9) are allfor illustration purposes and vary for different applications.

FIG. 5B illustrates a pair of identical symbols 520 according to anotherembodiment of the invention. The pair of identical symbols includes asymbol-0 522 and a symbol-1 524. The symbol-0 522 is initiallytransmitted and then immediately followed by the transmission ofsymbol-1 524. Each of the symbols 522 and 524 pertain to thetransmission of symbol A. In this embodiment, the symbol A includessamples A0-A13 which carry data values and a cyclic prefix associatedwith samples A10-Al3. The length of the cyclic prefix can correspond tomore or less than four samples as shown in FIG. 5B. Similarly, thenumber of samples A0-A13 for the symbol A can also vary depending uponimplementation. For example, if the multicarrier data transmissionsystem were an ADSL system, then the symbol might include a total of 512samples.

FIG. 5C illustrates a pair of identical symbols 540 according to yetanother embodiment of the invention. In this embodiment, the symbolsinclude both a cyclic prefix and a cyclic suffix. The pair of identicalsymbols 540 include symbol-0 542 and symbol-1 544. The symbol-0 542 andthe symbol-1 544 both pertain to the transmission of symbol A. In thisembodiment, the symbol A includes samples A0-A9 which carry data values,a cyclic prefix associated with samples A6-A9, and a cyclic suffixassociated with samples A0-A3.

It should be recognized, as noted above, that it is not necessary thattwo identical symbols be transmitted in succession. So long as the twoidentical symbols are preceded or succeeded by different symbols and thereceiver understands the manner by which the identical symbols will betransmitted, the receiver can analyze the pairs of identical symbols asreceived. It should also be noted that for a more robust estimation ofsymbol boundary timing, both the criteria of preceded and succeeded bydifferent symbols should be fulfilled. For example, the differentsymbols A, B, C and D can be transmitted as consecutive pairs (e.g., A,A, B, B, C, C, D, D). Even more general, a training sequence consists oftwo or more identical symbols where each contributing symbol is precededor succeeded by different symbols. For example, the symbols B, C, E, andF are all valid training symbols in the following transmitted trainingsequence: A, B, B, C, D, E, E, F, F, F, C, G. Note that in theseexamples, the identical symbols are preceded or succeeded by differentsymbols. Throughout the application, the example of using uniformconsecutive pairs (namely: A, A, B, B, C, C, D, D) is often assumed, theextension of the description to the general case is implicit in thisapplication.

FIG. 6 is a flow diagram of a symbol boundary determination process 600according to one embodiment of the invention. The symbol boundarydetermination process 600 stores 602 received training symbols in areceiver buffer. Then, the sample values of the received trainingsymbols stored 602 in the receiver buffer (which represent pairs ofidentical transmitted symbols) are processed to determine 604 differencevalues for a plurality of different choices of symbol boundary timing.Then, one of the choices is selected 606 to be the symbol boundarytiming based on the difference values. The difference values areassociated with a pair of received training symbols that were identicalon transmission. The difference values for such a pair of receivedtraining symbols provide an indication of interference. Theinterference, for example, pertains to inter-carrier or inter-symbolinterference. In other words, the difference values can representestimated noise due to inter-carrier and inter-symbol interference. Ifthe symbols preceding the identical pair of symbols are different,interference due to the preceding symbol is obtained. On the other hand,if the symbols succeeding the identical pair of symbols are different,interference due to the succeeding symbol is obtained. Hence, theselection 606 of one of the choices to be the symbol boundary timing canbe biased towards the difference values that are the lowest. The lowerthe difference value, the less interference is present using that symbolboundary timing. Consequently, the performance of the multi-carrier datatransmission system can be improved when the symbol timing boundary ischosen in view of interference levels.

Although the symbol boundary determination process 600 makes referenceto a pair of received training symbols, the difference values beingutilized to select the symbol boundary timing can be accumulated oraveraged over a number of pairs of symbols.

FIGS. 7A-7B are flow diagrams of a symbol boundary determination process700 according to another embodiment of the invention. The symbolboundary determination process 700 is, for example, performed by areceiver of a multicarrier data transmission system. As an example, thesymbol timing unit 208 within the receiver 200 illustrated in FIG. 2Bcan perform the symbol boundary determination process 700.

The symbol boundary determination process 700 initially clears 702 anydifference values (including any accumulated difference values) and sets704 variable x to zero (0). In addition, the variable n is set 706 tozero (0). Here, the operations 702-706 are used to initialize the systemfor the symbol boundary determination process.

Once the system has been initialized, a first sample value is retrieved708 from buffer position r(x+n). Here, the buffer position (r) pertainsto a position (x+n) within a receive buffer that holds a series ofincoming analog signals that were transmitted from a transmitter inaccordance with a training protocol. A second sample value is retrieved710 from buffer position r(x+symbol_length+1), where symbol_lengthrepresents the length of the symbols being transmitted to the receiver.Then, the first sample value is subtracted 712 from the second samplevalue to obtain a difference value. Typically, for ease ofimplementation, the subtraction 712 operation is implemented as anaddition (e.g., using two's complement values). Next, the magnitude ofthe difference value is determined 714. For example, the magnitude canbe determined by the absolute value of the difference value.Alternatively, another mathematical operation such as magnitude squarecan be determined for the difference value and used in place of themagnitude. The magnitude of the difference value is then added 716 to anaccumulated difference value. The difference values are accumulated overa plurality of different n values.

Next, a decision 718 determines whether the variable n=2N−1. Here, Nrepresents the number of subcarriers (e.g., tones) within a symbol. Itshould also be noted that the value of 2N−1 would be different in otherembodiments, such as embodiments that utilize a cyclic prefix or suffix.In any case, when the decision 718 determines that the variable n doesnot=2N−1, then the variable n is incremented 720. After the variable nis incremented 720, the symbol boundary determination process 700returns to repeat the operation 708 and subsequent operations so thatanother difference value can be obtained and accumulated for the currentbuffer offset specified by the variable x. On the other hand, when thedecision 718 determines that the variable n=2N−1, then all thedifference values have been accumulated within the accumulateddifference value for the current buffer offset identified by thevariable x. Accordingly, the accumulated difference value, representedas D(x), is stored 722.

Next, a decision 724 determines whether the variablex=2*symbol_length−1. Here, the decision 724 determines whether theoffset into the receive buffer has traversed a symbol length. When thedecision 724 determines that the buffer offset has not traversed asymbol length, the symbol x is incremented 726. Following the operation726, the symbol boundary determination process 700 returns to repeat theoperation 706 and subsequent operations so that an accumulateddifference value can be acquired for the next offset into the receivebuffer.

When the decision 724 determines that the variable x=2·symbol_length−1,then the accumulated difference values have been obtained for two symbollengths. Hence, a symbol boundary timing can then be determined 728based on the accumulated difference values (D(x)). Here, the symbolboundary timing can be determined 728 based on the offset (or timingindex) with respect to the receive buffer that offers the lowestaccumulated difference value. The search space over which the lowestaccumulated difference value is found is equal to a length of twosymbols. In the event that there are multiple accumulated differencevalues that are equally low, the difference value offering either theshortest or the longest offset into the receive buffer can be chosen,depending on the respective training sequence. Following the operation728, the symbol boundary timing has been determined and thus the symbolboundary determination process 700 is complete and ends. However, itshould be noted that the symbol boundary determination process 700 canbe performed multiple times for different pairs of first and secondsample value sets from the receive buffer and to accumulate or averagesuch values a plurality of times. In such case, the symbol boundarytiming can be based on the accumulation or average of the variousaccumulated difference values.

If the pairs of identical symbols in the training sequence are precededby the same symbols, there could be multiple accumulated differencevalues that are equally low. Since the inter-symbol interference due tothe preceding symbols is not taken into account with such a trainingsequence, the longest offset into the receive buffer can be chosen inorder to minimize the possible introduction of inter-symbol interferencedue to the preceding symbols.

If the pairs of identical symbols in the training sequence are succeededby the same symbols, there could be multiple accumulated differencevalues that are equally low. Since the inter-symbol interference due tothe succeeding symbols is not taken into account with such a trainingsequence, the shortest offset into the receive buffer can be chosen inorder to minimize the possible introduction of inter-symbol interferencedue to the succeeding symbols.

In other cases, the shortest offset into the receive buffer should bechosen as the received symbol boundary.

It should also be noted that, if the chosen offset into the receivebuffer is longer than the length of a symbol (symbol_length), the lengthof a symbol (symbol_length) can be subtracted from the chosen offset togive the received symbol boundary to improve latency.

FIG. 8 is a block diagram of a symbol boundary timing unit 800 accordingto one embodiment of the invention. The symbol boundary timing unit 800operates to determine a symbol boundary reference that is utilized by areceiver. For example, in accordance with one embodiment, the symbolboundary timing unit 800 can be utilized as the symbol timing unit 208illustrated in FIG. 2B.

The symbol boundary timing unit 800 receives incoming samples at areceive buffer 802. The incoming samples are analog signals from a wire.The receive buffer stores the samples and has enough capacity to storefour entire symbols. An adder 804 performs a subtraction operation withrespect to two different samples residing within the receive buffer 802.In one embodiment, the separation of the samples being supplied to theadder 804 with respect to the receive buffer 802 are separated by asymbol width (or an integer number of symbol widths). The output of theadder 804 is a difference value for the particular samples. Thedifference value can be a positive or negative value. Hence, thedifference value is supplied to an absolute value circuit 806 thatdetermines that absolute value (or magnitude) of the difference value.

Next, the difference value (i.e., its magnitude) is supplied to anaccumulator 808. The accumulator 808 accumulates difference values for2N−1 samples. The output of the accumulator 808 is an accumulateddifference value (e.g., D(x)). The accumulated difference value can alsobe referred to as a correlation value G(x). The accumulated differencevalue is supplied to a comparator 810. The comparator 810 determineswhether the accumulated difference value is to be utilized as the symbolboundary reference. In this regard, the comparator 810 can compare thecurrent accumulated difference value with one or more previouslydetermined accumulated difference values to select the particular one ofthe accumulated difference values that offers the best systemperformance. The comparator 810 may include a register to hold otherpreviously determined accumulated difference values or to hold thecurrently best accumulated difference value. The symbol boundary timingunit 800 also includes a controller 812. The controller 812 controlsoverall operation of the symbol boundary timing unit 800 and coordinatesthe operations with respect to at least the receive buffer 802, theaccumulator 808 and the comparator 810.

According to one aspect of the invention, a transmit sequence containingpairs of identical symbols is employed as the training protocol. Thisprotocol has pairs of identical symbols but the transmitted symbolspreceding or succeeding the pairs have to be different. Oneimplementation is to have a sequence of DMT symbols which areconstructed by a pseudo-random sequence and each individual DMT symbolis repeated once. By identifying the differences between the twoconsecutive symbols at the receiver end, noise due to inter-carrier andinter-symbol interferences can be estimated for different choices ofsymbol boundary timing. The choice with the least inter-carrier andinter-symbol interferences can then be selected for symbol framing. Thetraining protocol aims at minimizing inter-carrier and inter-symbolinterference, thus outperforms the other methods. It is independent ofthe value of Lcp, Lcs and β, where any of them are allowed to be zero.

A receive buffer at the receiver can store four symbols (e.g.,4×dmtsymbol_length) of time samples. The determined symbol boundary isdetermined at an optimum delay (md) from the beginning of the receivedbuffer in accordance with Equation 4 below: m_(d) = x  when$\begin{matrix}{{G(x)} = {\sum\limits_{n = 0}^{{2*{Nsc}} - 1}\quad{{abs}\left\lbrack {{r\left( {x + {{dmtsymbol}_{—}{length}} + n} \right)} - {r\left( {x + n} \right)}} \right\rbrack}}} & \left( {{Eq}.\quad 4} \right)\end{matrix}$

-   -   is minimized.        where r represents the received buffer contents, abs represents        taking the absolute value, and the search is performed over x=0        to 2·dmtsymbol_length−1. Further, the delay m_(d) is the        calculated optimum delay and is implemented as an address index        to the received buffer. For simplicity, a receive buffer of        length equal to four symbols of time samples is used as an        example to illustrate how the invention functions. However, the        length of such a receive buffer can be at least down to three        symbols of time samples if the use of a circular buffer is        considered.

Consider an example in which the FFT size is 1024, the number ofsubcarriers is 512, the length of the cyclic prefix (Lcp) is 48, thelength of the cyclic suffix (Lcs) is 48, and the length of anoverlapping region (β) is 16. Here, the dmtsymbol_length would be 1104(2·Nsc+Lcp+Lcs−β). Using such exemplary numbers, Equation 4 is rewrittenas: $\begin{matrix}{{G(x)} = {\sum\limits_{n = 0}^{1023}\quad{{{abs}\left\lbrack {{r\left( {x + 1104 + n} \right)} - {r\left( {x + n} \right)}} \right\rbrack}.}}} & \left( {{Eq}.\quad 4.1} \right)\end{matrix}$This calculation is done for x=0 up to x=2207. This means the symboldelay is searched over a space of two symbol lengths. Note that 80samples corresponding to the cyclic prefix and suffix are removed at thereceiver, thus leaving 1024 samples to be processed. Therefore, therewill be 2208 correlation values calculated for each pair of trainingsymbols, i.e., G(0), G(1), . . . , G(2207) are to be calculated fromEquation 4.1.

To explain further the operation of Equation 4.1, suppose we denote theabsolute difference signal as:d(n)=abs[r(1104+n)−r(n)].  (Eq. 4.2)In order to compute the 2208 correlation values, G(0), . . . , G(2207),there are 3231 absolute difference values required. The calculations ofthese absolute difference values are done as follows: $\begin{matrix}{{{d(0)} = {{abs}\left\lbrack {{r(1104)} - {r(0)}} \right\rbrack}}{{d(1)} = {{abs}\left\lbrack {{r(1105)} - {r(1)}} \right\rbrack}}{{d(2)} = {{abs}\left\lbrack {{r(1106)} - {r(2)}} \right\rbrack}}\vdots{{d(3230)} = {{abs}\left\lbrack {{r(4334)} - {r(3230)}} \right\rbrack}}} & \left( {{Eq}.\quad 4.3} \right)\end{matrix}$The signal samples r are stored in a receive buffer as shown in FIG. 8.

Each of the correlation values are then calculated with a slidingwindow. $\begin{matrix}{{{G(0)} = {{d(0)} + {d(1)} + \ldots + {d(1023)}}}{{G(1)} = {{d(1)} + {d(2)} + \ldots + {d(1024)}}}{{G(2)} = {{d(2)} + {d(3)} + \ldots + {d(1025)}}}\vdots{{G(2207)} = {{d(2207)} + {d(2208)} + \ldots + {d(3230)}}}{{{In}\quad{general}},}} & \left( {{Eq}.\quad 4.4} \right) \\{{G(x)} = {{d(x)} + {d\left( {x + 1} \right)} + \ldots + {{d\left( {x + 1023} \right)}.}}} & \left( {{Eq}.\quad 4.5} \right)\end{matrix}$The optimum symbol boundary is found at m_(d) if $\begin{matrix}{{{G\left( m_{d} \right)} = {\min\left\{ {{G(0)},{G(1)},\ldots\quad,{G(2207)}} \right\}}}{{{symbol}_{—}{boundary}} = m_{d}}} & \left( {{Eq}.\quad 4.6} \right)\end{matrix}$A direct architecture which implements Equation 4.6 is shown in FIG. 9.

The computation load due to the direct implementation architecture ishigh. For every correlation value G(x) to be calculated, the number ofaccumulations amounts to 1024. If the instantaneous absolute differencevalue is stored in a buffer (of size 1024 in this example or up to 2×Nscin general), the computation of a correlation value G(x) as shown inEquation 4.5 can be re-formulated by a running sum window algorithm:G(x)=G(x−1)+d(x+1024)−d(x)  (Eq. 4.7)

The architecture of such a computation efficient architecture is shownin FIG. 10. By having a correlation buffer that stores all thecorrelation values, additional options in choosing the correlation valuecan be used. For example, more robust search algorithms for the bestminimum value can be utilized.

Equation 4 uses an absolute value in forming the correlation valuesG(x). Alternatively, a magnitude square can be used. That is, thecorrelation values G(x) can be determined in accordance with thefollowing equation. $\begin{matrix}{{G(x)} = \left. {\sum\limits_{n = 0}^{{2*{Nsc}} - 1}\quad{abs}} \middle| {{r\left( {x + {{dmtsymbol}_{—}{length}} + n} \right)} - {r\left( {x + n} \right)}} \right|^{2}} & \left( {{Eq}.\quad 5} \right)\end{matrix}$

The result of the summation (of Equations 4 or 5) is essentially ameasure of the inter-symbol and inter-carrier interference. At anyspecific value of x, the received sample is calculated as:r(x+dmtsymbol_length+n)=wanted signal+random-noise(inter-carrier andinter-symbol interference)r(x+n)=wanted signal+random-noise(inter-carrier and inter-symbolinterference).Due to pairs of identical transmitted signals, the “wanted signal”between r(x+dmtsymbol_length+n) and r(x+n) are identical. Through thesubtraction, the “wanted signal” is cancelled out. Further, since theinterference results from random signals, the difference between the twointerference signals is still a measure of the interference. Therefore,by minimizing the result of the summation across 2*Nsc samples, oneobtains the symbol boundary with the least inter-carrier andinter-symbol interference.

The above-described approach to determining symbol boundaries can alsobe applied to minimize interference due to spectral leakage of crosstalkfrom other lines in a multiple-input multiple-output system. This isdone by having all transceivers, both near-end and far-end, transmitpairs of identical symbols. The determination of the symbol boundary canstill be done according to Equations 4 and 5, where the effect ofcrosstalk is automatically taken into account in the received timedomain samples.

The above-described approach to determining symbol boundaries continuesto apply even when transmitter windowing is utilized. Namely, Equations4 and 5 continue to apply. In order to improve the ability to determinethe boundary, according to one embodiment, the ideal window for symboltiming detection is a rectangular window with a length equal to onesymbol width (i.e., dmtsymbol_length).

When noise cancellation schemes are employed, their contribution shouldbe ignored during the estimation based on Equations 4 and 5. Forexample, in an echo-cancelled system, the transceiver detecting symbolboundary should be silent and only the far-end transmitters and othernear-end transmitters are active. The inter-symbol interference andinter-carrier interference from the far-end transmitter in the directchannel and from both near-end and far-end transmitters in neighboringlines can be measured according to Equations 4 and 5 and echo noise willnot be present during the estimation. This approach can give the optimalsymbol boundary as long as there is a large degree of echo-cancellation.Similarly, if the effect of near-end crosstalk (NEXT) from near-endtransmitters will be cancelled, the near-end transmitters should besilent during the estimation period.

In cases where the channel spread is longer than the cyclic prefix, atime domain equalizer or a per-tone equalizer can be used to shorten theeffective channel spread as known to those skilled in the art. For atime domain equalizer or a per-tone equalizer of T taps, Equation 4 canbe modified by reducing the range for summation as follows:m_(d) = x  when $\begin{matrix}{{\sum\limits_{n = 0}^{{2*{Nsc}} - T}\quad{{{abs}\left\lbrack {{r\left( {x + {{dmtsymbol}_{—}{length}} + n} \right)} - {r\left( {x + n} \right)}} \right\rbrack}\quad{is}\quad{minimized}}},} & \left( {{Eq}.\quad 5.1} \right)\end{matrix}$where r represents the received buffer contents, abs represents takingthe absolute value, and the search is performed over x=0 to2*dmtsymbol_length−1.

Where the channel spread is shorter than the cyclic prefix, there may bemultiple equivalent values of m_(d) where the inter-carrier andinter-symbol interference are the same. Since the effective channeldelay is dependent on the value of m_(d) chosen, m_(d) should beselected such that the effective channel delay is minimized. In thisperspective, if there exists a range of delay values which allcorrespond to the same level of inter-carrier and inter-symbolinterference, the smallest m_(d) value can be chosen. This, in turn,leads to a smaller value for the cyclic suffix needed and thus reducesthe amount of overhead.

FIG. 9 is block diagram of a symbol boundary determination system 900according to one embodiment of the invention. The symbol boundarydetermination system 900 represents one implementation of Equation 4noted above, which determines a symbol boundary.

The symbol boundary determination system 900 includes a receiver buffer902 that, in this embodiment, stores four DMT symbols. A symbol timingcontrol unit 904 coordinates operation of the symbol boundarydetermination system 900. In this regard, the symbol timing control unit904 supplies buffer enable control signals to the receiver buffer 902,and supplies buffer address pointer controls to the receiver buffer 902.One of the buffer address pointers is denoted r(x+n), and the otherbuffer address pointer is denoted r(x+dmtsymbol_length+n), where r isthe contents of the receiver buffer 902 at the particular address, xrepresents a search range from 0 to 2*dmtsymbol_length−1, and nrepresents the particular samples to be accumulated.

The samples of the receiver buffer 902 as indexed by the buffer addresspointers are supplied to an adder 906 that performs a subtractionoperation. The result from the adder 906 is supplied to an absolutevalue (abs) circuit 908. The output of the absolute value circuit 908 isa partial correlation value represented byabs(r(x+dmtsymbol_length+n)−r(x+n)). The partial correlation value issupplied to an adder 910 which adds the current partial correlationvalue with an accumulated correlation value produced by an accumulator912. The symbol timing control unit 904 also supplies a reset/enablesignal to the accumulator 912. The output of the accumulator 912 is acurrent correlation value G(x) which is supplied to a comparator 914.The comparator 914 selects a minimum correlation value from the currentcorrelation value supplied by the accumulator 912 with a previousminimum correlation value stored in register 916. The symbol timingcontrol unit 904 can send a minimum value control signal to thecomparator 914. In response to the minimum value control signal, theoutput of the comparator 914 is a symbol boundary value. For example,the symbol boundary value can be an optimum delay (md) from thebeginning of the receiver buffer 902, thereby specifying the symbolboundary.

FIG. 10 is a block diagram of a symbol boundary determination system1000 according to another embodiment of the invention. The symbolboundary determination system 1000 represents a more efficientcomputational scheme than that utilized by the symbol boundarydetermination system 900 illustrated in FIG. 9.

The symbol boundary determination system 1000 includes a receiver buffer1002 that stores four DMT symbols. A symbol timing control unit 1004supplies buffer addressing and control signals to the receiver buffer1002 so that a pair of samples can be retrieved from the receiver buffer1002 and supplied to an adder 1006. One of the samples supplied from thereceiver buffer 1002 to the adder 1006 is denoted r(x+n), and another ofthe samples supplied from the receiver buffer 1002 to the adder 1006 isdenoted r(x+dmtsymbol_length+n). The adder 1006 subtracts one of theretrieve samples from the other to produce a difference value. Thedifference value is supplied to an absolute value (abs) circuit 1008which produces a value. The output of the absolute value circuit 1008 isa partial correlation value represented byabs(r(x+dmtsymbol_length+n)−r(x+n)).

The value from the absolute value circuit 1008 is supplied to a runningsum window 1010. More particularly, the value from the absolute valuecircuit 1008 is supplied to an absolute value delay buffer 1012 and anadder 1014. The delayed value from the absolute value delay buffer 1012is supplied to an adder 1016. The adder 1016 also includes a sum fromthe adder 1014. The adder 1016 subtracts the delayed value from the sumsupplied by the adder 1014 to produce a modified sum that is supplied toan accumulator 1018. Once all of the accumulations have occurred, theoutput of the accumulator 1018 is a correlation value. That is, theoutput of the accumulator 1018 is supplied to the adder 1014 so that apartial correlation value can accumulate to produce a final correlationvalue. The final correlation value is applied to a correlation buffer1020. The correlation buffer 1020 can store a plurality of finalcorrelation values that are being produced by the symbol boundarydetermination system 1000. The symbol timing control unit 1004 alsosupplies control signals to the absolute value delay buffer 1012, theaccumulator 1018 and the correlation buffer 1020. The absolute valuedelay buffer 1012, the adder 1014, the adder 1016 and the accumulator1018 can be considered as the running sum window 1010.

The symbol boundary determination system 1000 also includes a comparator1022. The comparator 1022 can evaluate the final correlation valueswithin the correlation buffer 1020 to select a minimum correlationvalue. The minimum correlation value that is chosen by the comparator1022 is used to determine a symbol boundary indicated by a symbolboundary value. For example, the symbol boundary value can be an optimumdelay (m_(d)) from the beginning of the receiver buffer 1002, therebyspecifying the symbol boundary. The symbol timing control unit 1004 alsosupplies control signals to the comparator 1022 and a symbol boundaryvalue register 1024, which stores the selected final correlation valuechosen by the comparator 1022.

FIG. 11 shows an exemplary graph of correlation values plotted againstthe symbol delay indices. In this example, the size of FFT used is 1024and a typical 26 American Wire Gauge (awg) cable channel with a looplength of 2 km is modeled. The minimum correlation value is found at thedelay index equal to 147. That is, in this particular example,m_(d)=147. The modeling parameters as well as the cable model arereferred to in John A. C. Bingham, “ADSL, VDSL, and MulticarrierModulation,” Wiley, ISBN 0-471-20072-7, which is hereby incorporatedherein by reference.

The determination of the optimum symbol delay index is based on asequence of pseudo-random symbols containing identical pairs in order toreduce the statistical noise due to the correlation process. Equation 4only describes the correlation process of a training symbol pair. Toextend it for a sequence of identical training pairs, Equation 4 ismodified for the kth training pair as: $\begin{matrix}{{G\left( {x,k} \right)} = {\sum\limits_{n = 0}^{{2*{Nsc}} - 1}\quad{{{abs}\left\lbrack {{r\left( {x + {{dmtsymbol}_{—}{length}} + n} \right)} - {r\left( {x + n} \right)}} \right\rbrack}.}}} & \left( {{Eq}.\quad 6} \right)\end{matrix}$

The representative iterative calculation of the G(x, k) values for eachtraining pair is as follows. The FFT size assumed is 1024, the number ofsubcarriers is 512, the length of the cyclic prefix (Lcp) is 48, thelength of the cyclic suffix (Lcs) is 48, and the length of anoverlapping region (β) is 16. Here, the dmtsymbol_length would be 1104(2·Nsc+Lcp+Lcs−β). When the calculation of the last correlation value,i.e., G(2207), is finished, the address pointers which are used to pointto the receive buffer are reset to the addresses that contain r(0) andr(1104), which are the beginning addresses for calculating G(0) of the(k+1)^(th) training pair, namely, G(0, k+1). Hence, for N trainingsymbol pairs, the correlation values that are collected as a result ofapplying Equation 6 are: $\begin{matrix}\begin{matrix}{0^{th}{symbol}\quad{pair}\text{:}} & {G\left( {0,0} \right)} \\\quad & {G\left( {1,0} \right)} \\\quad & \vdots \\\quad & \vdots \\\quad & {G\left( {2207,0} \right)} \\{1^{st}\quad{symbol}\quad{pair}\text{:}} & {G\left( {0,1} \right)} \\\quad & {G\left( {1,1} \right)} \\\quad & \vdots \\\quad & \vdots \\\quad & {G\left( {2207,1} \right)} \\\vdots & \quad \\\vdots & \quad \\\vdots & \quad \\{k^{th}\quad{symbol}\quad{pair}\text{:}} & {G\left( {0,k} \right)} \\\quad & {G\left( {1,k} \right)} \\\quad & \vdots \\\quad & \vdots \\\quad & {G\left( {2207,k} \right)} \\\vdots & \quad \\\vdots & \quad \\\vdots & \quad \\{\left( {N - 1} \right)^{th}\quad{symbol}\quad{pair}\text{:}} & {G\left( {0,\left( {N - 1} \right)} \right)} \\\quad & {G\left( {1,\left( {N - 1} \right)} \right)} \\\quad & \vdots \\\quad & \vdots \\\quad & {{G\left( {2207,\left( {N - 1} \right)} \right)}.}\end{matrix} & \left( {{Eq}.\quad 7} \right)\end{matrix}$

To select an optimum delay index, m_(d), there are two approaches. Thefirst approach is to average the N collected correlation values at eachdelay index and then subsequently to select the minimum value. The indexthat corresponds to the minimum correlation value is the optimum delayindex. If we denote the average correlation value at delay index x as{overscore (G)}(x), then $\begin{matrix}{{{\overset{\_}{G}\left( m_{d} \right)} = {\min\left\{ {{\overset{\_}{G}(0)},{\overset{\_}{G}(1)},\ldots\quad,{\overset{\_}{G}(2207)}} \right\}}}{{{{symbol}_{—}{boundary}} = m_{d}},{{optimum}\quad{delay}\quad{index}}}} & \left( {{Eq}.\quad 8} \right)\end{matrix}${overscore (G)}(x) is calculated as $\begin{matrix}\begin{matrix}{{\overset{\_}{G}(x)} = {\sum\limits_{k = 0}^{N - 1}\quad{G\left( {x,k} \right)}}} \\{= {{G\left( {x,0} \right)} + {G\left( {x,1} \right)} + {G\left( {x,2} \right)} + \ldots + {G\left( {x,{N - 1}} \right)}}}\end{matrix} & \left( {{Eq}.\quad 9} \right)\end{matrix}$Alternatively, {overscore (G)}(x) can also be calculated as$\begin{matrix}\begin{matrix}{{\overset{\_}{G}(x)} = {\frac{1}{N} \cdot {\sum\limits_{k = 0}^{N - 1}\quad{G\left( {x,k} \right)}}}} \\{= {\frac{1}{N} \cdot \left\lbrack {{G\left( {x,0} \right)} + {G\left( {x,1} \right)} + {G\left( {x,2} \right)} + \ldots + {G\left( {x,{N - 1}} \right)}} \right\rbrack}}\end{matrix} & \left( {{Eq}.\quad 10} \right)\end{matrix}$

The second approach of finding the optimum delay index is done byidentifying the delay index of the minimum correlation value for eachtraining pair and followed by averaging the identified delay index overall training pairs. Hence, with reference to Equation 8, we firstperform $\begin{matrix}\begin{matrix}{0^{th}{symbol}\quad{pair}\text{:}} & {{G\left( {x_{0},0} \right)} = {\min\left\{ {{G\left( {0,0} \right)},{G\left( {1,0} \right)},\ldots\quad,{G\left( {2207,0} \right)}} \right\}}} \\\quad & {{m_{d}(0)} = x_{0}} \\{1^{st}\quad{symbol}\quad{pair}\text{:}} & {{G\left( {x_{1},1} \right)} = {\min\left\{ {{G\left( {0,1} \right)},{G\left( {1,1} \right)},\ldots\quad,{G\left( {2207,1} \right)}} \right\}}} \\\quad & {{m_{d}(1)} = x_{1}} \\{\quad\vdots} & \quad \\{\quad\vdots} & \quad \\{\quad\vdots\quad} & \quad \\{k^{th}\quad{symbol}\quad{pair}\text{:}} & {{G\left( {x_{k},k} \right)} = {\min\left\{ {{G\left( {0,k} \right)},{G\left( {1,k} \right)},\ldots\quad,{G\left( {2207,k} \right)}} \right\}}} \\\quad & {{m_{d}(k)} = x_{k}} \\\vdots & \quad \\\vdots & \quad \\\vdots & \quad \\{\left( {N - 1} \right)^{th}\quad{symbol}\quad{pair}\text{:}} & {{G\left( {x_{k},{N - 1}} \right)} = {\min\left\{ {{G\left( {0,{N - 1}} \right)},{G\left( {1,{N - 1}} \right)},\ldots\quad,{G\left( {2207,{N - 1}} \right)}} \right\}}} \\\quad & {{m_{d}\left( {N - 1} \right)} = {x_{N - 1}.}}\end{matrix} & \left( {{Eq}.\quad 11} \right)\end{matrix}$In the above equation, m_(d)(k), is the delay index corresponding to theminimum correlation values for the k^(th) symbol. $\begin{matrix}{{{{averaged}_{—}m_{d}} = \frac{x_{0} + x_{1} + \ldots + x_{N - 1}}{N}}{{symbol}_{—}{boundary}} = {{averaged}_{—}m_{d}}} & \left( {{Eq}.\quad 12} \right)\end{matrix}$

The length of the training sequence can vary for different cable looplengths and, in general, the shorter the cable loop length, the shorterthe training sequence that is required. Typically, as the cable looplength varies from 300 m (short loop) to 2 km (long loop), the requiredtraining sequence varies from 500 symbols (250 identical pairs) to 1500symbols (750 identical pairs).

Although the above-discussion concentrates on a time domain approach todetermining symbol boundaries, the invention is also suitable for use asa frequency domain approach. The frequency domain approach can, forexample, be used with ADSL or VDSL systems. The advantage of processingthe training symbols in the frequency domain is to allow a range ofinformation bearing subcarriers (tones) in addition to detecting thesymbol boundary with the identical pseudo-random symbols carried in thesubcarrier (tones) other than the information bearing subcarriers.

To extend this detection method, the training pairs in the frequencydomain must be modified and an example training pair in the frequencydomain is shown as follows:

-   -   symbol A0:        -   tone_(—)1: information bearing CC        -   tone_(—)2: ×2 (pseudo-random 4-QAM symbols)        -   tone_(—)3: information bearing DD        -   tone_(—)4: ×4 (pseudo-random 4-QAM symbols)        -   tone_(—)5: information bearing EE        -   tone_(—)6: ×6 (pseudo-random 4-QAM symbols)        -   tone_(—)7: information bearing FF        -   tone_(—)8: ×8 (pseudo-random 4-QAM symbols)    -   symbol A1:        -   tone_(—)1: information bearing CC        -   tone_(—)2: ×2 (pseudo-random 4-QAM symbols)        -   tone_(—)3: information bearing DD        -   tone_(—)4: ×4 (pseudo-random 4-QAM symbols)        -   tone_(—)5: information bearing EE        -   tone_(—)6: ×6 (pseudo-random 4-QAM symbols)        -   tone_(—)7: information bearing FF        -   tone_(—)8: ×8 (pseudo-random 4-QAM symbols)            In the above example, the DMT symbol contains only eight            tones, but for any practical system, many more tones shall            be used. For example, 255 tones are used in ADSL. It is            shown in the example that the even tones of the training            pair contain identical random signals and these are ×2, ×4,            ×6, and ×8, while all the odd tones contain system            information to be transmitted between the customer's            premises (CP) and the central office (CO). The same system            information may be carried by the training pair. Note that,            in general, any number of tones (including zero) may be used            for system information and a minimum of one tone should be            used as the training tone. The use of pseudo-random 4-QAM            symbols as the training symbol is also for illustrative            purposes and other training symbols could be used.

Recall Equation 4,${G(x)} = {\sum\limits_{n = 0}^{{2*{Nsc}} - 1}\quad{{abs}\left\lbrack {{r\left( {x + {{dmtsymbol}_{—}{length}} + n} \right)} - {r\left( {x + n} \right)}} \right\rbrack}}$wherein dmtsymbol_length is the length of a DMT symbol, and Nsc is thetotal number of subcarriers (an even number).

For a delay index x, two sequences in the frequency domain are defined,one, R0(x), corresponding to the term r(x+n), and the other, R1(x),corresponding to the term r(x+dmtsymbol_length+n).R0(x,k)=FFT{r(x+n),n=0,1,2, . . . ,2·Nsc−1}k=0,1,2, . . . ,2·Nsc−1  (Eq.13)R1(x,k)=FFT{r(x+dmtsymbol_length+n), n=0,1,2, . . . ,2·Nsc−1}k=0,1,2, .. . ,2·Nsc−1  (Eq. 14)

The correlation values between a training pair are calculated as:$\begin{matrix}{{G(x)} = {\sum\limits_{k\quad{\varepsilon I}}\quad{{f\left\lbrack {{R\quad 1(k)} - {R\quad 0(k)}} \right\rbrack}.}}} & \left( {{Eq}.\quad 15} \right)\end{matrix}$where I is the set of tone indices that contains the random symbols,e.g. 2, 4, 6, . . . , Nsc−2, and where the function f[.] can take on anabsolute value, i.e.f[.]=abs[.]; or a magnitude square function,i.e.f[.]=abs[.]². Note also that the sum is only performed over theindices defined for the set of the random symbol bearing tones, which isindicated as k ε I.

The symbol boundary is found by minimizing Equation 15 over all possibledelay index x. G(m_(d)) = min {G(0), G(1), …  , G(2207)}symbol_(—)boundary = m_(d)

Thus far, the selection of the symbol boundary for one training symbolpair is described. If there is a sequence of training symbol pairs, theapproaches discussed above for time domain detection also applies to thefrequency domain based approaches.

FIG. 12 illustrates a block diagram of an initial section 1200 of atransmitter according to one embodiment of the invention. The initialportion 1200 can replace an initial portion of the transmitter 100illustrated in FIG. 2A, whereby the initial portion 1200 would replacethe blocks 102 through 110 of the transmitter 100 as illustrated in FIG.2A. The initial portion 1200 includes a first switch 1202 and a secondswitch 1204. The second switch 1204 receives training symbols 1206 andsystem information 1208 as inputs. The second switch 1204 then selectsone of the training symbols 1206 or the system information 1208 as anoutput that is supplied to the first switch 1202. The first switch 1202also receives data 1210 that is input and converted by a line encoder1212 into encoded data which is supplied to the first switch 1202. Thefirst switch 1202 selects one of the encoded data and the output of thesecond switch 1204 as an output that is supplied to a DMT demodulationunit of the receiver. The initial portion 1200 also includes a trainingprotocol control unit 1214 that supplies control signals to the firstswitch 1202 and the second switch 1204. The training protocol controlunit 1214 controls the operation of the first switch 1202 and the secondswitch 1204 so that some tones contain system information, while othertones contain training symbols. Further, the training protocol controlunit 1214 controls whether to transmit training symbols or data from theencoder.

FIG. 13 illustrates a pair of representative training symbols 1300 inthe frequency domain according to one embodiment of the invention. Forexample, the pair of training symbols 1300 represents a training symbolpair frequency domain based processing. Note, odd tones contain systeminformation, while even tones contain training symbol information. Alsonote that each of the training symbols in the pair of training symbols1300 shown in FIG. 13 are identical. The pair of training symbols 1300illustrated in FIG. 13 is an example for a VDSL system with 512subcarriers (FFT size=1024).

It should be understood that the invention can generally permit trainingsymbols in the frequency domain to contain both training symbolinformation as well as other non-training data. In other words, ifdesired, only a subset of the tones need be used to carry trainingsymbol information. For example, as shown in FIG. 12, the second switch1204 can be controlled to interleave training symbol information withnon-training data. In the example shown in FIG. 13, odd tones oftraining symbols carry non-training data (e.g., system information),whereas even tones carry training symbol information. The trainingsymbol information can have a random or pseudo-random characteristic.However, the training symbols in the frequency domain can interleavetraining symbol information with non-training data in various other waysbesides alternating between even and odd tones.

FIG. 14 is a block diagram of a symbol boundary determination system1400 according to another embodiment of the invention. The symbolboundary determination system 1400 is suitable for use with frequencydomain based processing. In contrast, the symbol boundary determinationsystem 900 illustrated in FIG. 9 and the symbol boundary determinationsystem 1000 as illustrated in FIG. 10 are suitable for use in timedomain based processing.

The symbol boundary determination system 1400 includes a receiver buffer1402 that, in this embodiment, stores four DMT symbols. The receiverbuffer 1402 can be accessed by buffer addressed pointers and enablecontrol signals provided by a symbol timing control unit 1404. Theaddressed samples within the receiver buffer 1402 from a first DMTsymbol are supplied to a first Fast Fourier Transform (FFT) 1406, whilethe samples pertaining to a second DMT symbol within the receiver buffer1402 are supplied to a second FFT 1408. For example, with the FFT sizeat 1024, the first FFT 1406 receives samples r(x) through r(x+1023) andthe second FFT 1408 receives samples r(x+1104) through r(x+2127). TheFFTs 1406 and 1408 output complex values that are supplied to an adder1410 that performs a subtraction operation to produce a differencevalue. The difference value produced by the adder 1410 is supplied to anabsolute value (or absolute value squared) circuit 1412. The output fromthe absolute value circuit 1412 is a partial correlation value.

The partial correlation value is then supplied to an adder 1414. Theadder 1414 adds the partial correlation value with an accumulatedcorrelation value that is stored in an accumulator 1416. The sumproduced by the adder 1414 is then stored in the accumulator 1416 as anext accumulated correlation value. For example, in a system using 512subcarriers, the accumulation would be performed 511 times. A resetsignal is supplied by the symbol timing control unit 1404 to theaccumulator 1416. The output of the accumulator 1416 following theaccumulation period is a final correlation value G(x). The finalcorrelation value is supplied to a comparator 1418. The comparator 1418determines a minimum correlation value. In this regard, the comparator1418 receives a comparator control signal from the symbol timing controlunit 1404. The comparator 1418 determines whether the final correlationvalue provided by the accumulator 1416 is less than a previous minimumcorrelation value stored in a register 1420. After all of the finalcorrelation values have been processed, a minimum final correlationvalue is output by the comparator 1418 and used to set the symbolboundary based on a symbol boundary value (md).

The invention can be implemented by software, hardware or a combinationof hardware and software. The hardware can be custom circuitry,customized circuitry (ASIC) and/or general circuitry (e.g., digitalsignal processor). The invention can also be embodied as computerreadable code on a computer readable medium. The computer readablemedium is any data storage device that can store data which canthereafter be read by a computer system. Examples of the computerreadable medium include read-only memory, random-access memory, CD-ROMs,DVDs, magnetic tape, optical data storage devices, and carrier waves.The computer readable medium can also be distributed overnetwork-coupled computer systems so that the computer readable code isstored and executed in a distributed fashion.

The advantages of the invention are numerous. Different embodiments orimplementations may, but need not, yield one or more of the followingadvantages.

One advantage of the invention is the ability to accurately determinesymbol boundaries in a multicarrier data transmission system. Anotheradvantage of the invention is the ability to obtain higher datatransmission rates because symbol boundary timing can be chosen withregards to interference.

In one embodiment, a symbol boundary can be selected at the position atwhich the amount of interference is minimized. In this manner, thesymbol boundary timing can be optimized. As a consequence, the data ratethe multicarrier data transmission system can support can be increased.The invention is particularly useful in situations in which the systemhas a long loop.

Another advantage of the invention is that it can be implemented using atime domain or a frequency domain training sequence. Such flexibilityallows the symbol boundary training protocol to adapt to variousdifferent implementations.

Still another advantage of the invention is that the symbol boundarytraining protocol can make use of a non-data aided training sequence. Inother words, the receiver does not care what the transmitter is sending.As a result, the training of the receiver is more elegant and requiresless control.

The many features and advantages of the present invention are apparentfrom the written description and, thus, it is intended by the appendedclaims to cover all such features and advantages of the invention.Further, since numerous modifications and changes will readily occur tothose skilled in the art, the invention should not be limited to theexact construction and operation as illustrated and described. Hence,all suitable modifications and equivalents may be resorted to as fallingwithin the scope of the invention.

1. A method for detecting received symbol boundary timing in amulticarrier system, said method comprising: estimating noise due tointer-carrier and inter-symbol interference for different choices ofsymbol boundary timing; and selecting the choice of symbol boundarytiming with the least noise due to inter-carrier and inter-symbolinterference.
 2. A method as recited in claim 1, wherein themulticarrier data transmission system includes at least one transmitterthat transmits a training sequence such that at least two identicaltraining symbols are preceded or succeeded by different symbols.
 3. Amethod as recited in claim 2, wherein each of the training symbols beingtransmitted twice are transmitted in pairs.
 4. A method as recited inclaim 3, wherein the pairs are transmitted as consecutive pairs.
 5. Amethod as recited in claim 3, wherein the training symbols areQuadrature Amplitude Modulation (QAM) symbols or Phase Shift Keying(PSK) symbols.
 6. A method as recited in claim 1, wherein themulticarrier data transmission system is a digital subscriber linesystem.
 7. A method as recited in claim 6, wherein the training symbolsinclude at least one of a cyclic prefix and a cyclic suffix.
 8. A methodas recited in claim 1, wherein the training symbols have a random orpseudo-random characteristic.
 9. A method as recited in claim 1, whereinsaid selecting selects, as the selected symbol timing, the choice ofsymbol boundary timing so as to minimize the noise due to inter-carrierand inter-symbol interference.
 10. A method for detecting receivedsymbol boundary timing in a multicarrier system, said method comprising:(a) receiving a series of received training signals over a wire-basedchannel; (b) storing at least three of the series of received trainingsignals to a buffer; (c) determining difference values for a pair ofconsecutive received training signals stored in the buffer; (d)selecting one of the difference values; and (e) determining the receivedsymbol boundary timing based on the selected one of the differencevalues.
 11. A method as recited in claim 10, wherein said selecting (d)operates to select the smallest of the difference values.
 12. A methodas recited in claim 10, wherein the multicarrier data transmissionsystem is a digital subscriber line system.
 13. A method as recited inclaim 12, wherein the training symbols are Quadrature AmplitudeModulation (QAM) symbols or Phase Shift Keying (PSK) symbols.
 14. Amethod as recited in claim 13, wherein the training symbols include atleast one of a cyclic prefix and a cyclic suffix.
 15. A method asrecited in claim 10, wherein the difference values can be accumulated oraveraged over a plurality of different pairs of received trainingsymbols, and wherein said selecting (d) selects one of the accumulatedor averaged difference values.
 16. A method for detecting receivedsymbol boundary timing in a multicarrier system, said method comprising:(a) receiving a series of received training signals over a wire-basedchannel; (b) determining a plurality of correlation values for aplurality of pairs of received training signals, where each respectivepair of training symbols were identical when transmitted; (c) selectingone of the correlation values; and (e) determining the received symbolboundary timing based on the selected one of the correlation values. 17.A method as recited in claim 16, wherein the correlation valuesrepresent correlation values for each of the plurality of pairsaccumulated or averaged over the plurality of pairs.
 18. A method asrecited in claim 16, wherein said determining (b) of the plurality ofcorrelation values is performed with time domain processing.
 19. Amethod as recited in claim 16, wherein each of the correlation values isobtained in accordance with the following equation:${G(x)} = {\sum\limits_{n = 0}^{{2*{Nsc}} - 1}\quad{{abs}\left\lbrack {{r\left( {x + {{dmtsymbol}_{—}{length}} + n} \right)} - {r\left( {x + n} \right)}} \right\rbrack}}$where r represents buffer contents storing the received trainingsymbols, abs represents taking the absolute value, Nsc is the number ofsubcarriers, and the search is performed over x=0 to dmtsymbol_length−1.20. A method as recited in claim 16, wherein said determining (b) of theplurality of correlation values is performed with frequency domainprocessing.
 21. A method as recited in claim 16, wherein each of thecorrelation values is obtained in accordance with the followingequation:${G(x)} = {\sum\limits_{k \in I}{f\left\lbrack {{{R1}(k)} - {{R0}(k)}} \right\rbrack}}$where I is the set of tone indices that contains the training symbols,where the function f[.] is one of an absolute value and an absolutevalue squared function, and where, for a delay index x, two sequences inthe frequency domain are defined as follows:R0(x,k)=FFT{r(x+n),n=0,1,2, . . . ,2·Nsc−1}k=0,1,2, . . . ,2·Nsc−1R1(x,k)=FFT{r(x+dmtsymbol_length+n), n=0,1,2, . . . ,2·Nsc−1}k=0,1,2, .. . ,2·Nsc−1 where r represents buffer contents storing the receivedtraining symbols, Nsc is the number of subcarriers, and FFT represents afast Fourier transform operation.
 22. A multicarrier data transmissionsystem, comprising: at least one transmitter, said transmitter includingat least: a line encoder that encodes data into symbols to betransmitted; training symbols; a switch that selects symbols fortransmission either said training symbols or the symbols pertaining tothe encoded data from said line encoder; a multicarrier modulation unitfor modulating the symbols into modulated symbols; and adigital-to-analog converter for converting the modulated symbols intoanalog signals; and at least one receiver, said receiver including atleast: an analog-to-digital converter for converting the analog signalsinto digital samples; a symbol timing unit for examining the analogsignals to produce correlation values for a plurality of differentpotential symbol timing boundaries; a multicarrier demodulation unit fordemodulating the digital samples into symbols; and a line decoder thatdecodes the symbols into data, wherein said transmitter transmits pairsof identical training symbols, and wherein said symbol timing unitproduces the correlation values with respect to the pairs of identicaltraining symbols as received at said receiver.
 23. A multicarrier datatransmission system as recited in claim 22, wherein the correlationvalues pertain to estimated noise due to inter-carrier and inter-symbolinterference for the different potential symbol timing boundaries.
 24. Amulticarrier data transmission system as recited in claim 23, whereinsaid symbol timing unit selects the one of the different potentialsymbol timing boundaries with the least estimated noise due tointer-carrier and inter-symbol interference as a selected symbol timingboundary.
 25. A multicarrier data transmission system as recited inclaim 22, wherein said multicarrier data transmission system furthercomprises another switch that supplies symbols from either the trainingsymbols or system information to said switch, and wherein said switchthat selects symbols for transmission operates to select either thesymbols pertaining to the encoded data from said line encoder or fromthe symbols provided by said another switch.
 26. A multicarrier datatransmission system as recited in claim 25, wherein said transmitterfurther comprises a control unit, and wherein said switch and saidanother switch are controlled by said control unit.
 27. A multicarrierdata transmission system as recited in claim 22, wherein said anotherswitch is used to interleave training symbol information with the systeminformation.
 28. A multicarrier data transmission system as recited inclaim 22, wherein said symbol timing unit determines the correlationvalues for the plurality of different potential symbol timing boundariesbased on time domain processing.
 29. A multicarrier data transmissionsystem as recited in claim 22, wherein said symbol timing unitdetermines the correlation values for the plurality of differentpotential symbol timing boundaries based on frequency domain processing.30. A computer readable medium including at least computer program codefor detecting received symbol boundary timing in a multicarrier system,said computer readable medium comprising: computer program code forestimating noise due to inter-carrier and inter-symbol interference fordifferent choices of symbol boundary timing; and computer program codefor selecting the choice of symbol boundary timing with the least noisedue to inter-carrier and inter-symbol interference.